The chairman of the Senate’s Commerce Committee demanded answers Tuesday from Facebook founder Mark Zuckerberg over whether his operation censored conservative media by keeping topics popular with Republicans out of the ubiquitous online service’s list of trending topics.

The inquiry comes a day after Gizmodo reported that Facebook employees did suppress conservative news.

“Have Facebook news curators in fact manipulated the content of the Trending Topics section, either by targeting news stories related to conservative views for exclusion or by injecting non-trending content?” Sen. John Thune, chairman of the Commerce Committee, asked in a letter to Mr. Zuckerberg.

Mr. Thune said if Facebook presents its news section as neutral, but in fact does suppress conservative news, it would be misleading the public.

The trending topics are supposed to be driven by algorithms, but former employees told Gizmodo they saw conservative topics culled by “curators” based either on the topic or on the news outlet.

Mr. Thune asked Mr. Zuckerberg to explain how curators operate, and demanded a list of stories that were removed from or injected into the trending topics list since it started in January 2014.

Facebook, in a statement Monday, said its policies prohibit political bias, and do not ban any specific news outlets from appearing in the trending list.

Sen. Harry Reid, the top Democrat in the Senate, panned Mr. Thune’s probe, saying there were other things Congress could be doing with its time than demanding answers from Facebook.

“The Republican Senate refuses to hold hearings on Judge Garland, refuses to fund the President’s request for Zika aid and takes the most days off of any Senate since 1956, but thinks Facebook hearings are a matter of urgent national interest,” said Reid adviser Adam Jentleson. “The taxpayers who pay Republican senators’ salaries probably want their money back.”